1. Field of the Invention
The present invention relates to a process for preparing a solidified material containing a coal ash as a main component which can be suitably used as building materials such as panels for building, construction materials such as roadbed materials and crushed stones, artificial aggregates, and the like.
2. Description of the Related Art
In thermal power plants and various factories utilizing coal as an energy source, a coal ash such as fly ash is produced in large quantities. About 4,000,000 tons of coal ash are produced each year in Japan, and only about 40% of such amount of the coal ash is being effectively utilized. The remaining 60% of the coal ash is presently being disposed of. However, it is not always easy to secure a disposal place for the coal ash, and owing to the compensation of fishery rights and the establishment of recycling laws, finding a disposal place is more and more difficult. Therefore, increasing the effective utilization of the coal ash is an urgent theme.
Heretofore, it has been suggested that the coal ash be utilized as an inorganic building material, as disclosed in Japanese Patent Application Laid-open Nos. 17247/1988 and 305044/1992. As a peculiar example, it has been suggested that the coal ash be utilized as one material for artificial fish gathering places, as shown in a publication "Construction of an artificial submarine mountain with a coal ash", Nippon Kogyo Shinbun K. K., Feb. 26, 1993. Of these suggestions, in the case that the coal ash is utilized as a construction material such as the material for artificial fish gathering places and as a building material, the use of a large amount of the coal ash can be expected.
By the way, in the case that the above-mentioned coal ash is utilized as a construction material such as the material for artificial fish gathering places or as a building material, the coal ash is usually mixed with cement and then used in either case. However, since a hydration reaction is utilized, the mixing ratio of the coal ash is not always high, though the coal ash is used as the main material. If it is intended to increase the mixing ratio of the coal ash up to 40% by weight or more, there is a problem that the strength of an obtained solid deteriorates, and for this reason, the utilization of the coal ash is at most 40% by weight. Furthermore, in this case, a large amount of cement is used, and so there is another problem that the dimensional stability of the solid in water is poor. This can be understood to be due to the fact that alight (3CaO.cndot.SiO.sub.2), belight (2CaO.cndot.SiO.sub.2) and the like are contained in a cement clinker, and the hydration reaction is carried out between these components and the coal ash to produce a calcium silicate hydrate gel (C-S-H) and calcium hydroxide, but since the rate of this hydration reaction is slow, the unreacted alight, belight and the like remain in large quantities in the solid. In addition, a large amount of calcium hydroxide is produced by the hydration reaction, but this hydrate is considered to have a bad influence on the dimensional stability.
Here, "the dimensional stability of the solid in water is poor" means that a volume change of the solid in dry-wet states is large.
A technique can also be contrived in which a composition of the coal ash and the cement is cured in an autoclave, but when 40% by weight or more of the coal ash is mixed, a solid having sufficient strength cannot be obtained even with autoclave curing.
Furthermore, in the case of building materials, weight reduction is required in addition to strength enhancement and the improvement of water resistance, depending upon the purpose for which the material is to be used. Generally, for the sake of weight reduction, the building material has been prepared by curing a cement slurry containing a blowing agent in an autoclave, and the thus obtained material is excellent in fire resistance, heat insulating properties and sound insulating properties, and it is further lightweight. Thus, this kind of material has been partially utilized as an autoclaved light weight concrete (ALC) in panels for building of houses and the like. Such a coal ash also has been partially utilized as the lightweight building material in ALC and the like. However, its mixing ratio is not always high, and when the mixing ratio of the coal ash is increased up to 40% by weight or more, the strength of the lightweight solid deteriorates. In addition, it also has the drawback that the dimensional stability to water is poor.
With regard to the utilization of the coal ash solid as the ALC, since the conventional ALC has a large porosity, there is a problem that a reinforcement material such as iron reinforcing rods is necessary in the case that the coal ash is used in, for example, a floor material which requires strength. Thus, it has been desired to develop ALC having sufficient strength even without using any reinforcement material.
In view of the above-mentioned problems of the conventional techniques, the present invention has been made, and an object of the present invention is to provide a solidified material containing a coal ash in a high mixing ratio and having a good dimensional stability to water, a high strength and a slight unevenness. Another object of the present invention is to provide a solid having weight reduction properties in addition to the above-mentioned characteristics.